Nowadays, many imaging devices such as multifunction printers, copying machines and scanners are provided with an automatic document feeder (ADF) for automatically transporting individual sheets from a stack of media sheets to an image reading position, and then ejecting and restacking the sheets automatically. Typically, an optical image reader is arranged at the image reading position to read (i.e. scan) the image on one side of the media sheet. Conventional automatic document feeders also have a double-sided (“duplex”) mode wherein images on both sides of a sheet are scanned by the image reader. In the double-sided mode, after the image on one side of a sheet is scanned by the image reader, the sheet is partially discharged, and then the same sheet is re-routed back through the ADF so that the image on the opposite side of the sheet can be scanned.
Maximizing the throughput performance of an ADF requires the ability to begin picking up a subsequent media sheet from a stack of media sheets while the previously picked media sheet is at the image reading position. In some situations, image quality is more important than high throughput, and it is more desirable to wait until a sheet has been completely scanned by the image reader before picking up a subsequent sheet. The challenge is to provide an ADF that can accommodate more than one timing sequence for picking so as to give the user a choice between high throughput and high image quality. Conventional low-cost automatic document feeders are not capable of providing both double-side scanning and flexible timing sequence for picking.